Advances in maternal-infant HIV prophylaxis with antiretroviral therapy (ART) have dramatically reduced mother to child transmission (MTCT), but yearly, >200,000 new vertical HIV infections (HIV+) occur, predominantly in resource poor nations. With potent early ART initiation, HIV-infected (HIV+) infants can survive into adulthood. The role and nature of immune mechanisms that are important for HIV reservoir establishment and mechanisms of HIV persistence in HIV+ infants on ART are poorly understood and may differ in infants compared to adults. HIV infection stimulates an immune response that can be protective and have detrimental effects as well. The infant immune system undergoes dynamic developmental changes and is also challenged by vaccines to stimulate immunologic memory. T follicular helper cells (Tfh) are a unique subset of CD4 TCM cells that home to germinal centers (GC) in lymph nodes (LN) and facilitates antibody responses of B cells following vaccination, and this subset has been demonstrated in adults as a major HIV reservoir. We hypothesize that establishment of HIV reservoirs and HIV persistence in infancy are influenced by host immune response, timing of ART initiation and events such as childhood immunizations that stimulate immunologic memory. This proposal will perform investigations to assess latent and active reservoirs in peripheral blood in the context of a developing immune system prospectively in HIV+ infants starting ART at age <2 mo. in Maputo, Mozambique. Peripheral Tfh (pTfh), which are CD4 TCM subset with partial phenotypic and functional similarity to Tfh in LN will be investigated in infants given childhood vaccines. An older-aged HIV+ Rome cohort, (age 5y-18y) who started ART within age <1 year, either early (<6mo) or late (7-12mo) and remained consistently aviremic will provide complementary information about immunity and reservoirs in early versus late treated children after periods of durable viral control on ART. State-of-art tools including 15 color flow cytometry, RNA Sequencing, Fluidigm BioMark platform for transcriptomics of fractionated cells and droplet digital PCR for HIV reservoir are among techniques to be used for three specific aims: 1. To investigate immunologic biomarkers of HIV reservoirs pre-and post-ART in HIV+ infants starting ART at age <2 mo. and in older- aged children who started ART at different times <1 year of age. 2. To investigate the effect of childhood vaccinations on HIV reservoirs in HIV infected infants with early ART initiation and to evaluate their vaccine responses in comparison with EUI infants; 3. To investigate gene signatures in antigen-stimulated memory T cells including pTfh post-vaccination to ascertain the relationship between vaccine responses and HIV reservoirs. The proposed studies could provide insights that build new hypotheses, develop biomarkers of HIV reservoirs for selecting patients best suited for ?cure? related clinical trials, and lead to innovative therapeutic strategies for eradication of active and latent HIV reservoirs for durable HIV remission.